The giant rocks, named 2012 QG42 and 2012 QC8, were first spotted by the robotic telescopes of the Catalina Sky Survey in Arizona on August 26, 2012. They have been speeding toward Earth and the moon and will be reaching their closest point tonight.

Asteroid QC8 is about 0.62 mile (one kilometer) wide and will be about 5.4 million miles (8.7 million kilometers) away at its closest approach, or 23 times the distance the moon is from Earth. At 625 to 1,400 feet wide (190 to 430 meters), QG42 is smaller, but it will get considerably closer—making it a more significant event for sky-watchers.

QG42 is roughly the size of a 14-story building and is officially classified as a potentially hazardous asteroid (PHA) by astronomers because of its proximity.

"We know of roughly 1,700 PHAs of this size or larger, and one of this size is expected to strike the Earth every 40,000 years on average, with an equivalent impact energy of 140 megatons of TNT," Yeomans said.

"The resulting crater would have a diameter of about 1.9 miles [3 kilometers]."

In the few hours when QG42 will be at its closest and brightest, sky-watchers will need at least a 12-inch telescope to hunt it down in the skies. But SLOOH—an Internet-based space tracking service—will be keeping an eye on the asteroid with its robotic telescopes on the Canary Islands and will broadcast the flyby live over the Web, starting at 7 p.m. ET.

More than 90 percent of Earth-orbit crossers larger than 0.62 mile (one kilometer) have been documented, and none represent a near-term threat, Yeomans said. So attention has now turned to hunting down near-Earth objects larger than 459 feet (140 meters)—like QG42.

"Roughly 40 percent of this population has been found, and none represent a threat," Yeomans said.

"Consequently, more than 95 percent of the risk of near-Earth asteroid collisions has been retired."